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Astrophotography

Exploring the Hubble Sequence by Eye

M100, a tightly wound spiral galaxy in the Virgo Cluster, was imaged by the Hubble Space Telescope to measure the universe's expansion rate. Amateurs won't see anywhere near this much detail, but with patience and care they should be able to classify the galaxy on the basis of its appearance in the eyepiece.

Courtesy John Trauger (JPL) and NASA.

Images taken with large telescopes show that no two galaxies are alike and that the variety of galactic forms is endlessly fascinating. Indeed, photographs from Mount Wilson's great reflectors inspired Edwin Hubble to order galaxies along the lines of his now-famous “tuning-fork” sequence of galaxy types. The visual observer, however, is at the mercy of the eye's limited angular resolution (20 or 30 arcminutes) at low light levels. Thus only the broadest categories of galaxies can be differentiated. Nevertheless, by noticing all you can about a galaxy's structure, you can make a fairly reliable guess about its classification, or Hubble type. The origin and physical significance of galaxy types remains an area of active research, and they have yet to be completely accounted for in most professionals' eyes.

Amateurs and the Hubble Sequence

Of course, amateurs with telescope apertures of 16 inches or larger will be able to observe structural features better and, in exceptional cases, to make a detailed visual classification. Anyone who has viewed M51 in Canes Venatici, or the southern galaxy NGC 1365, in a large telescope has readily seen some spiral structure. But there's more to classifying galaxies than spotting spiral arms. The key parameters are (1) the relative brightness of the bulge or core of the galaxy to that of the outer disk (if any) and (2) the rate at which the brightness decreases outward from the nucleus.

These 15-arcminute-wide views from the Digitized Sky Survey (DSS) show 12 of the galaxies that Brian Skiff studied at the eyepiece in a quest to characterize their Hubble types visually. The DSS enables anyone with Internet access to view small portions of wide-angle photographs taken at Palomar Observatory and elsewhere. The galaxies appear in the order mentioned in the text. North is up with east to the left.

Courtesy the Association of Universities for Research in Astronomy.

In the uncommon pure elliptical galaxies, of course, there is no disk. The lenticulars (type S0) and “early”-type spirals (type Sa) have relatively weak disks and pronounced bulges. As one progresses to “later” galaxy types, cores grow fainter and fainter, until one reaches type Sm, where the last vestige of a bulge remains as a tiny starlike nucleus. The Magellanic-type irregular galaxies are defined as having no nucleus at all: whereas ellipticals are “all bulge,” Magellanic irregulars are “all disk.”

Elliptical and disk galaxies are also distinguished by the way the galaxy light falls off from the center to the edge. Spheroidal bulges decline rapidly in brightness (as the inverse fourth power of the radius, for the mathematically inclined). When combined with the eye's peculiar perception of brightness (Sky & Telescope: March 1990, page 311), this makes elliptical galaxies appear to fade rather smoothly from the center outward, regardless of telescope aperture. A spiral galaxy's disk, by contrast, fades much more slowly, and visually it can seem a dull, uniform patch surrounding the concentrated core.

Southern Hemisphere observers can study the morphology of these galaxies with minimal optical aid. The Large Magellanic Cloud (left) and the Small Magellanic Cloud (right) define the Magellanic class of irregular galaxies; they are small satellite systems of our Milky Way. The Large Cloud has a prominent bar that can easily be perceived with the aid of binoculars.

Courtesy Akira Fujii.

Although galaxies of all types are scattered across the sky, it is convenient as an exercise in galaxy observing to have the Virgo Cluster close by (as these things go! — the cluster's distance is roughly 60 million light-years). Most of the richest galaxy clusters contain only elliptical and lenticular galaxies — no spirals. But Virgo, which barely makes the grade as a rich cluster by some definitions, contains every type of galaxy appearing in the standard classification schemes. It is convenient also to have a variety of galaxies in close proximity to one another on the sky so comparisons can be made quickly, sometimes in the same eyepiece field. Finally, a collection of galaxies all at roughly the same distance lets us compare the relative brightnesses of various Hubble types directly.

Examining Ellipticals

The realm of the galaxies, as seen by Northern Hemisphere observers on spring evenings. The boxed region, on the Virgo-Coma Berenices border, is shown on the next page.

Courtesy Akira Fujii.

What most observers consider to be "downtown Virgo" is centered on the region including Markarian's Chain, starting with M84 and M86, and sweeping northeast to M88, which is somewhat detached from the rest (Sky & Telescope: May 1994, page 42). All the galaxies in Markarian's Chain are early types (ellipticals and lenticulars), and it is natural to start by looking at M84 and M86, easily the most prominent objects at the chain's southwestern end.

On the second Palomar Observatory Sky Survey (POSS-II),M84 looks like a pure elliptical galaxy without a hint of a disk. This agrees with the online NASA Extragalactic Database, which lists M84's type as E1. Various observations I have made over the years with different telescopes indicate the brightness rising smoothly and quickly toward the center, where a conspicuous but nonstellar nucleus resides. The outer boundary seems to be indefinite because it merges so smoothly with the sky background. On the POSS-II film (digitized portions of which are available here), M86 shows a weak but distinct collar of brightness in the inner regions and a huge oval corona in the outermost parts — larger even than that of M87, which some consider to be the dominant member of the Virgo Cluster. These two features signify the type S0-, the “earliest” lenticular following the pure ellipticals. NED also considers the galaxy to be a transition case.

Telescopically M84 and M86 appear outwardly similar, but they differ subtly in their inner regions. While M84 fades without a break until lost to the sky's background glow, M86 has a zone where the brightness nearly levels off for an arcminute or two in radius. This indicates one can discern the disklike attributes of lenticular galaxies even in a small telescope. Both galaxies have nuclei that rise sharply from the bright background of their inner regions. If you observe them at quite high magnification, say 200x on an 8-inch telescope, can you tell any difference between them?

More Ellipticals

This region, on the Virgo-Coma Berenices border, is shown in a photograph that suggests the level of detail that might be visible with a midsize telescope under clear, dark skies. Click on the image to see all the galaxies in this area.

Courtesy Roberto Crippa.

Look now at the oval halo of M86. At a dark site even my 70-millimeter Pronto refractor will show it reaching nearly as far as a 12.5-magnitude star 4.2' southeast of the galaxy's center. This implies a size of about 8' by 5' — consistent with the values tabulated on page 7.

M84 and M86 form two points of a triangle including the somewhat fainter galaxy NGC 4388 to the south. In the center of that triangle is another even fainter galaxy, NGC 4387. This is a more typical lenticular member of the Virgo Cluster and provides a good example of the contrast in sheer size between galaxies that lie at the same distance.

Before leaving the early-type galaxies, it is worth slipping about a degree southeast to M87 and its several companions. M87 is usually called a “giant elliptical” (type E+), and images or photographs reaching to very low light levels show the galaxy to have an extensive but faint corona similar to that of M86. Visually M87 is another case like M84, where the brightness fades without a break until it merges with the sky. How large does the galaxy appear? (For a “yardstick,” note that 5.7 arcminutes separate M87's core and HD 108915, an 8.5-magnitude star due north of the galaxy.)

M87's two companions to the southwest, NGC 4476 and 4478, are again smaller and fainter examples of lenticular and elliptical galaxies. More interesting to me is NGC 4486B, which lies just west of HD 108915. At lower powers in a modest telescope, NGC 4486B looks like a 13.5-magnitude star. But by cranking up the magnification one can see that the object is not quite starlike; rather it resembles a small, high-surface-brightness planetary nebula a few arcseconds across. This is an excellent example of a “compact elliptical” (cE) galaxy. The brightness profile of a compact elliptical is identical in form to that of a giant elliptical, but compact ellipticals are as much as a hundred times less luminous than giant ones. The brightest example of a compact elliptical — it defines the class, in fact — is M32, the Andromeda Galaxy's most conspicuous companion. M32's core is so small it defies resolution (that is, it remains starlike) even with the Hubble Space Telescope (Sky & Telescope: March 2000, page 22).

Scrutinizing Spirals

Another spring treat for Northern Hemisphere observers, this trio of bright spiral galaxies in Leo illustrates another telltale characteristic of some spirals: an elongated profile that narrows because our sightline nearly crosses the plane of the galaxy's disk. Clockwise from top are NGC 3628, M65, and M66.

Courtesy Akira Fujii.

The remaining galaxies in the Markarian Chain are all early-type galaxies. NGC 4461 is a lenticular whose prominent bar and rings we view obliquely. NGC 4477, toward the chain's northeast end, is a face-on lenticular that shows some spiral structure.

M88 is our first definite spiral galaxy (type Sb), and a beauty it is! Medium-size telescopes show a fairly bright oval halo surrounding a moderately large and somewhat brighter oval core. Higher powers best show the very small nucleus. These three components of the visual impression are common to ordinary spiral galaxies. Ellipticals and many lenticulars lack this three-part halo-core-nucleus zonation; instead, they fade from center to edge without any kind of plateau. Relative to the galaxy as a whole, M88's nucleus is not as prominent as those of the lenticulars, but is it brighter or fainter than they in absolute terms? Switch views among some of the similarly bright Markarian Chain lenticulars and compare for yourself.

About 50 arcminutes to the east of M88 lies a barred spiral galaxy, M91. With a Hubble type of SBb, M91 is comparable in a sense to M88, but it shows a larger and brighter bulge relative to the arms, as well as a prominent bar. The difference between the two galaxies is evident even with my 70-mm Pronto, in which M91's bar appears elongated nearly east-west within a faint circular halo. The bar was even more readily visible to Stephen James O'Meara in a 4-inch refractor, according to his 1998 guidebook, The Messier Objects.

Spiral galaxies M99 (left) and M100 (right) sketched by Ronald J. Buta in 1977 with a 30-inch reflector at McDonald Observatory in Texas. This large telescope was necessary to unambiguously reveal the galaxies' spiral arms. The sketch below shows what can be seen through a smaller telescope. North is up with east to the left.

A few degrees to the north and west, two more Messier objects provide additional glimpses of spiral structure and the halo-core-nucleus pattern. M99 is of type Sc, and telescopic images show clumpy arms that contain bright star clouds and H II regions (nebulous regions of gas, driven to glow by the ultraviolet light from hot, young stars). The arm extending south from the center clearly stands apart from the main body of the galaxy as it wraps around to the west (as opposed to the innermost arm on the other side of the galaxy, which is much more tightly wrapped). A weak disturbance in the otherwise smooth glow was apparent to me in my 70-mm telescope. Those with 8- and 10-inch telescopes at dark sites

Spiral galaxies M99 (left) and M100 (right) sketched through a 4-inch TeleVue telescope in 1999 by Sky & Telescope contributing editor Stephen James O'Meara. Even a 4-inch (under dark skies) can isolate features that differentiate a spiral galaxy from an elliptical one. North is up with east to the left.

will see the detached arm readily at high power, and larger apertures will reveal a wealth of detail. However, with just about any aperture you should be able to note the relative size and brightness of the circular core (which ranks halfway between those of M91 and M88) and the extensive but relatively faint halo.

More regular in appearance is M100, a bright face-on spiral north of the most crowded parts of the Virgo Cluster. Given its Hubble type (Sbc) we expect to see a small, modestly bright core, which M100 delivers. Interestingly, however, M100 lacks a conspicuous nucleus, as I've noted in several observations I've made with telescope apertures up to 12 inches. Larger telescopes again reveal a number of subtle spiral features, particularly at high power.

At the Ragged End of the Hubble Sequence

Late-type spirals often sport large knots of star formation, which sometimes bear their own NGC numbers. Because NGC 4395 was the number given to the nucleus of this Canes Venatici spiral, that number is attributed to the galaxy as a whole, even though NGC 4401 is arguably more conspicuous.

Courtesy Allan Sandage (Carnegie Institution) and NASA.

Late-type spirals bright enough to be readily visible in other than wonderful viewing conditions will necessarily be closer than Virgo. Our best bet is farther north among the galaxies in the Canes Venatici group, at roughly half to two-thirds the distance of Virgo. We've already mentioned M51, which you might compare with M100 in Virgo. Among the latest-type spirals that can be considered bright is NGC 4395, which has a type of Sm, the last stage before irregular.

Although it has a total magnitude of 10.2, making it one of the brightest galaxies in the whole sky, NGC 4395's average surface brightness is roughly 10 times lower than those of the Messier objects in the Virgo Cluster. This makes it a challenge even to find, though from a dark site in my 70-mm telescope it can be seen, albeit as a diaphanous "almost nothing." Telescopes in the 12- to 16-inch range will reveal numerous faint clumps in the weak spiral arms, three of which have NGC numbers of their own. In NGC 4395 we see essentially a pure disk galaxy, where the central bulge is all but invisible, though photographs show the galaxy's nucleus as a weak stellar point. (How large a telescope is required to spot this diminutive bulge by eye?)

Irregular galaxies exhibit a wide range of star-formation rates, and luckily for aficionados of galaxy morphology it is easy to find irregulars with plenty of star formation to make them bright. Two examples reside in the Canes Venatici group.

NGC 4214 and NGC 4449 are similar in size, shape, and brightness, showing numerous giant H II regions across their surfaces. Both are bright enough to be spotted in hand-held binoculars as tiny spots, but naturally they become more interesting with larger telescopes. With a 16-inch it should be easy to isolate some of the glowing H II regions by viewing the galaxies at moderate power (that is, by using an eyepiece that yields an exit pupil of about 2 mm). Use a nebula filter that favors both the hydrogen-beta and [O III] (doubly ionized oxygen) emission lines, which are conspicuous in the spectra of interstellar gases that have been energized by newborn stars. The H II regions are particularly easy to see across the face of NGC 4449.

While the various ordinary spirals I've mentioned share a common three-zone structure, these two irregulars seem somehow unlike them, even visually. Compare these objects to the bright spirals near them, such as M63 and M106, to see the distinction more clearly.

Hubble-Sequence Examples in the Northern Spring Sky

Without a big telescope you probably won't be making detailed galaxy classifications when you step up to the eyepiece. But by paying attention to the characteristics of different galaxies on photographs and images and by noting how these characteristics manifest themselves visually, you will gain some of the skills that let you get the best out of any view of the sky.

Hubble-Sequence Examples in Northern Spring Sky

Galaxy

(2000.0)
R.A. Dec.

Mag. (V)

Size

Hubble Type

NGC 4214

12h 15.6m

+36° 20'

9.7

7.9' x 6.3'

Irr

M99

12h 18.8m

+14° 25'

9.8

5.4' x 4.8'

Sc

M100

12h 22.9m

+15° 49'

9.4

6.9' x 6.2'

Sbc

M84

12h 25.1m

+12° 53'

9.3

5.0' x 4.4'

E1

NGC 4395

12h 25.8m

+33° 33'

10.2

12.9' x 11.0'

Sm

M86

12h 26.2m

+12° 57'

9.2

7.4' x 5.5'

S0-

NGC 4449

12h 28.2m

+44° 06'

9.4

5.1' x 3.7'

Irr

NGC 4461

12h 29.0m

+13° 11'

11.2

3.7' x 1.5'

SB0

NGC 4477

12h 30.0m

+13° 38'

10.4

4.0' x 3.5'

SB0

NGC 4486B

12h 30.5m

+12° 29'

13.3

0.5' x 0.4'

cE

M87

12h 30.8m

+12° 24'

8.6

7.2' x 6.8'

E+

M88

12h 32.0m

+14° 25'

9.5

6.9' x 3.9'

Sb

M91

12h 35.4m

+14° 30'

10.2

5.4' x 4.4'

SBb

* Data from Sky
Catalogue 2000.0, Vol. 2 (2nd ed.), except for Hubble types, which are
provided by the author. All but four of these galaxies appear on chart B1
(the Virgo Cluster chart) of Sky Atlas 2000.0, 2nd ed. NGC 4395,
NGC 4214, and NGC 4449 appear on Chart 7 of Sky Atlas 2000.0. NGC
4486B appears on Chart 194 of Uranometria, Vol. 1, and on chart 725
of Millennium Star Atlas.

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